This invention relates in general to an electrostatic printing device, and in particular to an electrostatic printing device capable of printing grey tones. The electrostatic grey tone printing device is particularly useful in reproducing video pictures from a video signal.
Electrographic recording systems are well-known in the art and are typified by the type of systems shown in U.S. Pat. Nos. 3,611,419, 3,653,065 and 3,662,396. These electrographic recording systems utilize a series of recording electrodes and complementary electrodes. By proper application of voltages to the recording and complementary electrodes, a latent image may be formed on a dielectric paper which is moved past the electrodes. This latent image is then developed by well-known techniques in the art. The recording electrodes and complementary electrodes may be positioned either on the same side of the dielectric paper or on opposite sides of the dielectric paper. The recording electrodes and complementary electrodes are mounted in a device referred to as a writing head. The reproduction of a video picture by means of a multiple electrode writing head and a dielectric paper requires the ability to reproduce shades of grey. One approach is to vary the voltage on the recording electrodes in proportion to the desired density of black thereby producing a shade of grey; however, this process is unstable and gives unreliable results. As soon as the voltage on the recording electrodes is decreased, there is a risk of reaching the threshold where the ionized discharge in the air gap due to the surface roughness is not maintained. This threshold is unstable and varies with the roughness of the dielectric paper and also with other factors, such as the humidity of the air. If the voltage is increased and if a multiplexed electrode addressing system is used, there is a risk of producing ghost images.
Another approach for producing shades of grey is to provide a square matrix of a certain number of dots, for example four dots by four dots and to fill this matrix by a number of printed dots proportional to the desired level of grey. The drawback to this process is to sacrifice the picture resolution. In this example, if electrodes of 250 by 250 microns are used, the resolution of four dots per millimeter becomes one dot per millimeter. In order to recover the initial resolution, the number of electrodes should be multiplied by four, and the dimensions should be reduced to 62.5 by 62.5 microns. However, electrodes of these dimensions cause great difficulties in construction and significantly increased costs in manufacturing.